1. Field of the Invention
This invention relates to a sensor system and a method for locating transmitters.
2. Discussion of Prior Art
It is known to locate transmitters with an antenna which is scanned until its signal is a maximum; this gives transmitter bearing relative to the system but not intervening distance. The distance can be determined by two steerable directional antennas of known separation and relative orientation: this gives two different transmitter bearings with a known baseline from which transmitter/system distance can be calculated. It suffers from the disadvantage of requiring two steerable directional antennas, which should ideally be spaced apart by a distance comparable to that between the transmitter and location system. If spurious multipath signals are present, adequate accuracy may not be obtainable.
Pulse-echo target location systems (radar, sonar, lidar) are also well known. They employ directional antennas and determine target distance from pulse time of flight and direction from bearing of peak receive signal. Features in the system field of view reflect interrogating pulses irrespective of whether or not they are transmitters.
In applications such as surveillance, there is a requirement for locating a transmitter to an accuracy of better than 5 m in a cubic space of side 100 m. Current radars and direction finding systems either lack sufficient accuracy or are undesirably expensive and complex, and can be difficult to mount on moving platforms of convenient size, eg road vehicles or aircraft.
U.S. Pat. No. 5,835,060 to Czarnecki et al discloses a long base line interferometer system for transmitter location. The system employs antennas at each end of the base line and measures phase differences between antenna output signals at successive positions along a system movement path between 2 m and 100 m long. Discontinuities in phase measurement are removed by xe2x80x9cunwrappingxe2x80x9d, ie addition or subtraction of 2xcfx80 to produce phase values between xcfx80 and xe2x88x92xcfx80. To remove an unwanted unknown phase constant, each successive measured phase difference is subtracted from the next along the measurement path to give a difference of differences or differential. Differentials are also predicted, for grid points being searched. Predicted differentials are then subtracted from measured equivalents to produce residuals; a cost function is then derived which is the sum of the squares of the residuals reduced to their principle value. The cost function with the lowest value is the starting point for non-linear least squares convergence to obtain a value for transmitter position, which can in turn be used as a new starting point for a further iteration. Unfortunately, this technique is sensitive to noise; moreover, as will be described later in more detail, simulation indicates that it is not sufficiently accurate, particularly at low signal to noise ratios.
It is an object of the invention to provide an alternative form of sensor system.
The present invention provides a sensor system for transmitter location incorporating:
a) two receiver elements responsive to incident radiation by generation of respective signals,
b) a processing system for determining phase difference data for pairs of element signals,
c) means for measuring sensor system position in terms of position data;
d) computer apparatus for determining transmitter position from phase difference data measured from processed element signals and calculated from trial transmitter locations,
characterised in that the computer apparatus is arranged to locate transmitters from magnitude or phase of circular functions of differentials between measured and calculated phase difference data.
The invention provides the advantage that because circular functions are used it is not necessary to alter phase values by 2xcfx80 or to calculate iteratively. Instead location is obtained directly. Moreover, simulation indicates that improved accuracy and noise immunity is obtained compared to the prior art.
The processing system may be arranged to process a pair of element signals or a pair of frequency downconverted signals equivalent thereto by multiplying one signal of each pair in either case by a complex conjugate of the other to enable their phase difference to be measured. It may be arranged to determine phase difference by:
a) mixing each signal of a pair with sine and cosine reference signals to determine in-phase and quadrature components,
b) multiplying each component of one signal by both components of the other to produce an in-phase component product, a quadrature component product and two products of in-phase and quadrature components,
c) adding the in-phase component product to the quadrature component product, and
d) subtracting one product of in-phase and quadrature components from the other.
The processing system may be arranged to digitise signals at a sampling rate prior to mixing with reference signals, the reference signals have a frequency of one quarter of the sampling rate, and mixing is implemented by multiplication of alternate samples by and one other sample in four by xe2x88x921.
The circular functions of differentials between measured and calculated phase difference data may be complex exponents, and the computer apparatus may be arranged to determine actual transmitter location by summing exponents over a range of system positions and to indicate transmitter location from the magnitude or phase of this summation. The computer apparatus may alternatively be arranged to determine actual transmitter location by producing summations of the exponents over a range of system positions, to multiply the summations together to form a product and to indicate transmitter location as that corresponding to a predetermined magnitude or phase of this product.
The measuring means may comprise a GPS base station and co-located with the receiver elements a GPS subsidiary station for co-operation with the base station and provision of position data.
The sensor system may be movable relative to a transmitter to be located, the base and subsidiary GPS stations being arranged to provide position data and the computer apparatus being arranged to subtract calculated phase differences from those of processed element signals for a series of sensor system positions.
The measuring means may be arranged to implement inertial navigation.
The system of the invention may incorporate at least three receiver elements disposed to define a plurality of measurement dimensions in which to locate a transmitter, and the receiver elements may be patch antennas.
In an alternative aspect, the present invention provides a method of locating a transmitter having the steps of:
a) providing two receiver elements responsive to incident radiation by generation of respective signals,
b) determining phase difference data for pairs of element signals,
c) measuring sensor system position in terms of position data;
d) determining transmitter position from phase difference data measured from processed element signals and calculated from trial transmitter locations,
characterised in that transmitter position is determined from magnitude or phase of at least one circular function of a differential between measured and calculated phase difference data.
Phase difference data may be determined for pairs of element signals by multiplying one signal of each pair in either case by a complex conjugate of the other to enable their phase difference to be measured: this may be implemented by:
a) mixing each signal of a pair with sine and cosine reference signals to determine in-phase and quadrature components,
b) multiplying each component of one signal by both components of the other to produce an in-phase component product, a quadrature component product and two products of in-phase and quadrature components,
c) adding the in-phase component product to the quadrature component product, and
d) subtracting one product of in-phase and quadrature components from the other.
Signals may be digitised during processing at a sampling rate prior to mixing with reference signals, the reference signals have a frequency of one quarter of the sampling rate, and mixing is implemented by multiplication of alternate samples by 0 and one other sample in four by xe2x88x921.
Phase differences may be calculated for a plurality of possible transmitter locations and actual transmitter location determined from correlation between calculated and measured phase differences.
The at least one circular function may be at least one complex exponent. It may be a plurality of complex exponents, actual transmitter location being determined by producing a summation of exponents over a range of system positions and transmitter location being that associated with a predetermined magnitude or phase of this summation. Actual transmitter location may alternatively by determined by producing a plurality of summations of exponents over a range of system positions and corresponding to respective dimensions of transmitter location, multiplying the summations together to form a product and indicating transmitter location to be that corresponding to a predetermined magnitude or phase of this product.
Position data is provided by means of a GPS base station co-operating with a GPS subsidiary station co-located with the receiver elements.
The receiver elements may movable relative to a transmitter to be located, position data may be provided by base and subsidiary GPS stations and calculated phase differences may be subtracted from those of processed element signals for a series of sensor system positions.
The method may employ at least three receiver elements in the form of patch antennas disposed to define a plurality of measurement dimensions in which to locate a transmitter.